1. Field of the Invention
The present invention relates to a method for forming photoresist patterns in the fabrication of a semiconductor device, and more particularly to a photomask for forming a high resolution photoresist pattern, capable of attenuating a light exposure energy with its strength concentrated on the center of the mask pattern being formed.
2. Description of the Prior Art
For clearly defining an outline with a minimum linear width by use of a photoresist film in a photo-masking process, there have been efforts to improve the resolution. These efforts have led to various process developments such as development of photoresist films exhibiting good characteristics and reduction of the wavelength of the light emitted from a light source or other light exposure equipment. However, the developed techniques can not solve a problem that the photoresist pattern exhibits a limited resolution due to an intrinsic wave surging property of the light source.
A conventional photomask for forming a photoresist pattern will now be described, in conjunction with FIGS. 1A to 1C.
As shown in FIG. 1A, the photomask has a chrome pattern 2 formed on a reticle 1 and is identical to a pattern for forming a contact hole. The photomask has an energy distribution that the energy peak is exhibited at the center of a portion of the photomask where the chrome pattern 2 is not present, as shown in FIG. 1B. In the photomask, energy is distributed symmetrically with respect to the energy peak point and gradually reduced toward opposite lateral edges of the photomask. As a result, a considerably low light exposure energy strength is exhibited at the lateral edges of the chrome pattern 2, thereby considerably degrading the resolution.
In other words, formation of a photoresist pattern using the above-mentioned conventional photomask involves a degradation in resolution because the photoresist film is partially left along predetermined boundaries between the chrome and the non-chrome (a'and b'in FIG. 1C).